Portable devices depend upon batteries as a power source. As the power consumption demands of these electronics devices is continually increasing, so is the need for high capacity batteries with long storage life. Depending upon the chemistry, rechargeable batteries comprise different characteristics. For example, one of the earliest rechargeable chemistries, Nickel Cadmium (Ni-Cad), are very robust and inexpensive, but have very poor capacity to weight ratios that result in large bulky batteries. Another later technology, Nickel Metal Hydride (Ni-MH), proved to be smaller, but offered no significant increase in the capacity to weight ratio.
Lithium, including Lithium Ion (Li-Ion) or Lithium Polymer (Li-P), batteries made a substantial leap in energy capacity per unit weight. With the advent of lithium cells, batteries became much smaller with greater capacities, as well as gaining superior storage capacity characteristics. The downside of lithium, however, involves safety. If lithium batteries are charged improperly or over charged, they can rapidly release gas at excessive temperatures, which can cause fire or explosion. Therefore, lithium battery chargers must precisely regulate the charging of such batteries. This precise charging regulation, however, in no way diminishes the need for very rapid charging algorithms.
Charging voltage must be precisely controlled in lithium battery chargers. If too much voltage is applied, battery cycle life will be degraded, and safety concerns may arise. There is thus a need for more accurate voltage sensing means in lithium battery charging systems.
In addition to a battery cell, lithium battery packs may also include memory devices. The memory devices are typically Electrically Programmable Read Only Memory (EPROM). These devices contain information that is used by the charger including identification, cycle data, etc. Also, battery packs contain thermistors for sensing temperature. Typically, lithium battery packs utilize four contact terminals: a positive voltage terminal, an EPROM data terminal, a thermistor terminal and a return terminal. Any means of more accurate voltage sensing, in order to me commercially viable, must add no new terminals to batteries.
It has been shown that accurate sensing of the cell voltage enhances the safety of charging lithium batteries. In addition, it has been shown that accurately measuring cell voltage reduces battery charge time by as much as 40%.
The net result is the need for an improved battery charging system.